Microelectronic or optoelectronic package having a polybenzoxazine-based film as an underfill material

ABSTRACT

Microelectronic and optoelectronic packaging embodiments are described with underfill materials including polybenzoxazine, having the general formula:

BACKGROUND OF THE INVENTION

[0001] 1). Field of the Invention

[0002] This invention relates to microelectronic and optoelectronicpackages and, more specifically, to an underfill material in suchpackages.

[0003] 2). Discussion of Related Art

[0004] A microelectronic or an optoelectronic package usually includeseither a microelectronic semiconductor die or an optoelectronic diemounted to a package substrate. The package substrate provides rigidityto the overall package, and often also provides electronic interconnectbetween the die and another device. An underfill material is introducedbetween the die and the package substrate.

[0005] The semiconductor die, for example, may have conductive memberson a lower surface thereof, and are positioned on top of an uppersurface of the package substrate. The conductive members provideelectric communication between the semiconductor die and the packagesubstrate. The conductive members also mount the conductor die to thepackage substrate. The conductive members are, however, relativelyfragile. Differences in coefficients of thermal expansion (CTE) betweenthe semiconductor die and the package substrate cause relative shrinkageor expansion between the semiconductor die and the package substrate.The underfill material serves to reduce stresses on the conductivemembers due to such relative expansion or contraction.

[0006] A filler material is usually included in the underfill material.Filler loading is usually required to reduce shrinkage during curing, tocontrol CTE, and to control moisture uptake. Filler loading tremendouslyincreases the viscosity of the film at bonding temperature. Theincreased viscosity tremendously increases the bonding force per bumpand reduces the wetting properties of melted film to all contactsurfaces. The increase of bonding force per bump often cracks the chipduring chip placement, and poor wetting properties of melted filmadversely affect the joint integrity and introduces voids that shortenthe joint fatigue life.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The invention is described by way of examples with reference tothe accompanying drawings, wherein:

[0008]FIGS. 1a-e are cross-sectional side views of polybenzoxazine-basedfilms according to different embodiments of the invention; and

[0009]FIGS. 2a-c are cross-sectional side views of electronic assemblieshaving polybenzoxazine-based underfill materials, according to differentembodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0010] Microelectronic and optoelectronic packaging embodiments aredescribed with underfill materials including polybenzoxazine, having thegeneral formula:

[0011] wherein n can be any integer from 1 to 20 and is preferably aninteger from 1 to 4 and is most preferably 1 or 2, and R₁ can behydrogen, one or more groups selected from hydroxyls, one or more linearor branched alkyls of 1 to 80 and more preferably 1 to 10 carbon atoms,aromatics, alkyl substituted aromatics, aromatic substituted alkyls of 6or 20 carbon atoms, mono or poly fluorine substituted alkyls of 1 to 20carbon atoms, a mono or poly fluorine substituted compound having atleast 1 aromatic ring and 6 to 20 carbon atoms, and phenolic compoundsof 6 to 20 carbon atoms (for the purpose of this specification phenoliccompounds may have more than one hydroxyl group as defined in chemicaldictionaries) including poly phenolic compounds having on average 6 to20 carbon atoms per phenol group. When n is 2, 3 or 4, R₁ can beselected from the connecting groups below:

[0012] Depending on whether phenolic or phenoxy repeat units are desiredin the polybenzoxazine, it may be desirable that R₁ be ortho, meta, orpara to the oxygen atom of the benzoxazine monomer of Formula A.Further, R₂ may be an alkyl of 1 to 40 carbon atoms, an aromatic, analkyl substituted aromatic or aromatic substituted alkyl of 6 to 40carbon atoms, mono or poly fluorine substituted alkyl of 1 to 20 carbonatoms, a mono or poly fluorine substituted compound having at least onearomatic ring and 6 to 20 carbon atoms or an amine of 1 to 40 carbonatoms including polyamines and aromatic, alkyl substituted aromatic, oraromatics substituted alkyls having 6 to 40 carbon atoms. Furthermore,each benzene ring, as shown by (R₃), where p is an integer from 0 to 3and R₃ is as defined later, can have more than one substituent of thesame structure or a mixture of the R₃ structures.

[0013] The variable m can be an integer from 0 to 5 and R₃ can be H orR₂. Preferably R₃ is not the amine or polyamine components of R₂.Preferably R₃ is an alkyl of 1 to 9 carbon atoms such as CH₃, C₂H₅,C₃H₇, or C₄H₉, or a mono or poly fluorinated alkyl of 1 to 9 carbonatoms such as CF₃, C₂F₅, or C₃F₇. These R₁ compounds are well known tothose familiar with phenolic compounds. Generally R₁ can be any of theknown connecting groups than interconnect two or more phenols. Knownconnecting groups refers to those which are present in commerciallyavailable phenols, are in experimentally available phenols, and phenolswhose synthesis are described in the published literature. Examples ofsuch phenols include:

[0014] As the cationically initiated ring opening polymerization ofmonofunctional benzoxazines generally results in linear polymers, it isdesirable that at least 25 mole percent, more desirably at least 50 molepercent and preferably at least 75 mole percent or 90 mole percent ofthe R₁ groups are not an additional hydroxyl groups or a phenolic orpolyphenolic compound and n in Formula A is 1. Also desirably at least25 mole percent, more desirably at least 50 percent, still moredesirably at least 75 or 90 mole percent of the R₂ groups are neitherpolyamines nor include additional benzoxazine compounds. Theselimitations are desirable for thermoplastic polybenzoxazines, but it isunderstood that if thermoset polybenzoxazines are desired, the amount ofdifunctional or polyfunctional benzoxazine monomers (those where n is 2to 20 or R₂ is a polyamine or includes additional benzoxazine rings)could be higher. When high molecular weight thermoplasticpolybenzoxazines are desired, desirably the number average molecularweight of the polybenzoxazine is at least 5,000 and more desirably atleast 10,000.

[0015] As is well known, benzoxazine monomers are made from the reactionof three reactants, aldehydes, phenols, and primary amines by proceduresusing a solvent or known as solventless systems. U.S. Pat. No. 5,543,516sets forth a generally solventless method of forming benzoxazinemonomers. An article by Ning and Ishida in Journal of Polymer Science,Chemistry Edition, vol. 32, page 1121 (1994) sets forth a procedureusing a solvent which can be used to prepare benzoxazine monomers. Theprocedure using solvents is generally common to the literature ofbenzoxazine monomers.

[0016] The preferred phenolic compounds are phenol or cresol, but caninclude diphenols (e.g., bisphenol-A), triphenols, etc., e.g.,polyphenols, wherein each phenolic group in the phenolic compound has onaverage about 6 to about 20 carbon atoms per phenol group. The use ofphenols with two or more hydroxyl groups reactive in formingbenzoxazines may result in branched and/or crosslinked products. Thegroups connecting the phenolic groups into a phenol can be branch pointsor connecting groups in the polybenzoxazine.

[0017] The aldehydes used to form the benzoxazine can be any aldehyde,but preferably the aldehydes are those having from about 1 to about 10carbon atoms, with formaldehyde being highly preferred. The amine usedto form the benzoxazine can be an aromatic amine, an aliphatic amine, analkyl substituted aromatic, or an aromatic substituted alkyl amine. Theamine can also be a polyamine, although the use of polyamines will,under some circumstances, yield polyfunctional benzoxazine monomers.Polyfunctional benzoxazine monomers are more likely to result inbranched and/or crosslinked polybenzoxazines than monofunctionalbenzoxazines, which would be anticipated to yield thermoplasticpolybenzoxazines.

[0018] The amines generally have from about 1 to about 40 carbon atomsunless they include aromatic rings, and then they may have from about 6to about 40 carbon atoms. The amine of di or polyfunctional may alsoserve as a branch point to connect one polybenzoxazine to another.

[0019] In the past, thermal polymerization has been the preferred methodfor polymerizing benzoxazine monomers. The temperature to induce thermalpolymerization is typically varied from about 150 to about 300° C. Thepolymerization is typically done in bulk, but could be done fromsolution or otherwise. Catalysts, such as carboxylic acids, have beenknown to slightly lower the polymerization temperature or accelerate thepolymerization rate at the same temperature.

[0020] Cationic polymerization initiators described in this inventionhave been found to result in polymerization of benzoxazine monomers attemperatures as low as cryogenic temperatures. Preferred temperaturesare from about −100° C. to about 250° C., and most preferably betweenabout −60 and 150° C. for ease of handling the reactants and products.Some of the cationic initiators, e.g., PCl₅, form repeating units fromthe benzoxazine monomers, that include a salt of the amine. Theserepeating units have better solubility in polar solvents, e.g., water,than similar repeating units without the amine salt. The initiators ofthe current invention can be used either in the benzoxazine melt or inthe presence of solvent, allowing the solvent content to be from 0% tonearly 100%. Many solvents can be used in cationic polymerizations, andtheir selection is known by those skilled in the art of cationicpolymerization.

[0021] The polymers from the cationically initiated polymerization ofbenzoxazine are useful as molded articles with good thermal stabilityand/or flame resistance, such as molded circuit boards, flame resistantlaminates, or other molded articles, and is a source of precursor tohigh temperature resistant chars. The common uses for high temperatureresistant chars include aircraft brake discs, equipment for sinteringreactions, and heat shields or heat shielding material. The polymerswhich include repeating units having amine salts can be used inapplications for partially or fully water soluble polymers such asviscosity control agents.

[0022] Generally, given optimal reaction conditions, cationic initiatorscan polymerize benzoxazine monomers or oligomers. These include H₂SO₄,HClO₄, BF₃, AlCl₃, t-BuCl/Et₂AlCl, Cl₂/BCl₃, AlBr₃, AlBr3.TiCl₄, I₂,SnCl₄, WCl₆, AlEt₂Cl, PF₅, VCl₄, AlEtCl₂, and BF₃Et₂O. Preferredinitiators include PCl₅, PCl₃, POCl₃, TiCl₅, SbCl₅, (C₆H₅)₃C⁺(SbCl₆)—,or metallophorphyrin compounds such as aluminum phthalocyanine chloride,which are all known to result in similar polymers from cationicallyinitiated polymerization of unsaturated monomers. Methyl iodide (acovalent initiator), butyl lithium (an ionic initiator), and benzoylperoxide (a radical initiator) were not effective at polymerizingexperimental benzoxazine monomers.

[0023] Methyl tosylate, methyl triflate, and triflic acid appear tocationically polymerize the experimental benzoxazine monomers, althoughthe polymers did not precipitate from solution. Typically, eachinitiator initiates a polymer with from about 3 to about 1,000 to 3,000repeat units, so the amount of initiator needed on a mole percent basisrelative to the monomer is small. However, an additional initiator isneeded to compensate for loss due to adventitious moisture and otherreactants that deactivate cations, that may be present in the monomerssolvents, etc. Desirably about 0.001 to about 50 mole percent initiatorbased upon the monomer and more desirably from about 0.01 to about 10mole percent initiator is used for these cationically initiatedpolymerizations.

EXAMPLES

[0024] Several factors have been found to change the structure of thebenzoxazine polymers prepared by cationic polymerization. These factorsinclude polymerization temperature, the particular cationic initiator,and competing reactivity between the ortho-carbon of the benzene ringand the basic nitrogen atom of the oxazine ring.

[0025] For example, the following results were obtained with PCl₅ as theinitiator at −60° C. and 40° C.:

[0026] Thus, the formation of a phenolic repeating unit having an aminesalt therein occurs preferentially to this kind of monomer(monoparasubstituted benzene ring other than the two sites occupied bythe oxazine ring) at low temperatures (e.g., −60° C.) with a PCl₅initiator. A phenoxy repeating unit occurs more frequently at 40° C.than at −60° C., but still is not the exclusive repeating unit. Theamine salt form of the phenolic repeating unit causes an increased watersolubility. Further, at −60° C. the availability or reactivity of anortho CH position also affects the relative ratio of phenolic to phenoxyrepeat units. Steric hindrance around the ortho CH position or asubstituent in the ortho Cl position favors a phenoxy structure from thebenzoxazine, while the absence of steric hindrance and absence of anortho substituent favors a phenolic structure. A unified theory toexplain the above is that there is a competition between the reactivityof the aromatic carbon (ortho with respect to the oxygen) and the basicnitrogen atom of the (O—CH₂—N(R₂)CH₂). When substitution on the aromaticring is in such a way that it reduces the reactivity of the ring orthocarbon, the protonation or alkylation of the nitrogen dominates and aphenoxy structure results. However, if the ortho CH position (withrespect to the O) is open and no bulky substituent offers sterichindrance, the ring reactivity dominates and a phenolic structure at lowtemperatures is the result. In summary, it is anticipated that at −60°C., if i) aromatic ring reactivity dominates, then a phenolic repeatingstructure is obtained, and if ii) oxazine ring reactivity dominates,then a phenoxy repeating structure is obtained, and that monomers givenbelow will result in an increased probability of the repeatingstructures given. Monomer Preferred Repeating Structure

Phenolic structure

Phenoxy structure

Phenoxy structure

Mixture phenolic and phenoxy

[0027] As phenolic repeat units from benzoxazine are now available fromlower temperature cationic polymerizations, it is desirable todistinguish the resulting polymer from thermally polymerized benzoxazinepolymers. Desirably the number average molecular weight of polymers fromcationically polymerized monofunctional benzoxazine monomers are atleast 2,000 or 5,000. Desirably the cationically polymerized benzoxazinewhen both phenolic and phenoxy structures result has at least 2, 5, 10,or 15, or at least 50, 80, or 90 mole % repeating units of the phenoxystructures. However, the pure phenolic structure when the molecularweight is above 2,000 or 5,000 is not excluded.

[0028] Five benzoxazine monomers were used for most of theexperimentation on cationically initiated polymerization. They were BA-amonomer and C-m monomer. Different chemical structures for two of themonomers are shown below:

BA-a monomer

[0029]

C-n monomer

[0030] BA-a monomer is made from the reaction of bisphenol A,formaldehyde, and aniline. C-m monomer is made from the reaction ofcresol, formaldehyde, and methylamine. C-m monomer was preparedaccording to the procedure set forth by Ning and Ishida in Journal ofPolymer Science, Chemistry Edition, vol. 32, page 1121 (1994). Thepolymerization solvents used were 1,2-dichloroethane,1,2-dichlorobenzene, chloroform, or deuterated chloroform. All thesolvents were dried by conventional methods and distilled under argon.The cationic initiators used in Table 1 were purchased from AldrichChemical Company and used without further purification. All solvents andreactants were stored in a dry box unless refrigeration was required.The moisture was usually less than 1 ppm in the dry box. Table 1 belowshows polymerizations, which were ran for about 20-80 hours using thespecified variety of cationic and other initiators. Table 1 sets forththe results of polymerizations of BA-a monomer. While run numbers 8, 9,and 11 did not yield any insoluble polymer, the solutions turned deepred and a tough red polymer was recovered when the solvent wasevaporated. The initiator amount was typically about 5 mole percentbased upon the amount of benzoxazine monomer: TABLE 1 ChCl₃ insoluableas wt. % based Temper- on total Run No. Initiator ature monomer weight 1Phosphorus pentachloride 20 56.2 (PCl₃) 2 Phosphorus trichloride (PCl₃)20 19.6 3 Phosphorus oxychloride 20 15.0 (POCl₃) 4 Titanium (V) chloride(TiCl₅) 19.4 5 Triphenylcarbenium 20 8.7 antimonatehexachloride [(C₆H₅)₃C+ (SbCl₆)⁻] 6 Antimony pentachloride 53 10.1 (SbCl₅) 7 Antimonypentachloride/ 20 12.3 oxetane (Sb Cl₅) 8 Methyl tosylate (MeOTs) — — 9Methyl triflate (MeOtf) — — 10 Aluminum phthalocynanine 20 20.4 chloride(metallophorphyrin) 11 Triflic acid — — 12 Borontrifluoro dietherate 530 13 Borontrifluoro dietherate/ 20 0 promoter 14 p-tolyl triflate 53 015 Methyl iodide covalent 53 0 initiator 16 Butyl lithium (anionic) 20 017 Benzoyl peroxide (free 110  0 radical)

[0031] TABLE 2 Char yield at Initiator Tg by DSC (° C.) 800° C. by TGA(%) PCl₅ 215 50.26 PCl₃ 216 48.93 POCl₃ 210 50.53 TiCl₅ 222 61.78Metallophorphyrin 186 43.59 MeOTf 193 31.48 MeOTs 142 28.06 Triflic acid— 31.29 (C₆H₅)₃C⁺(SbCl₆)⁻ — — SbCl₅ — — Thermal-cured (Cntrl) 177 28.56

[0032] Several of the polymers polymerized according to the conditionset forth in Table 1 were analyzed by thermogravimetric analysis (TGA)or differential scanning calorimity (DSC). The glass transitiontemperature (Tg) is used to characterize a particular polymer andmicrostructure. The char yield is affected by many factors, a high charyield is typically desired for precursors for char or for non-flammablepolymers. As can be seen in Table 2, the first four cationic initiatorsyielded polymers from benzoxazine having significantly higher Tg's thanthe last sample, which was a control (thermally cured) polybenzoxazinefrom the same monomer. The char yield from the polymers initiated withthe first four cationic initiators varied from about 44 or 49 to about62 weight percent based upon the weight of the initial startingpolybenzoxazine. This was significantly higher than the control, whichyielded only about 29 weight percent char under identical conditions.The next three cationic initiators (5-7) yielded polymers with slightlydifferent Tg's than the control. The char yield from the fifth throughthe seventh cationic initiators was also different than that of thecontrol, although the difference from the control amount was not assignificant as with the first four cationic initiators. It is to benoted that the polymer from the methyl tosylate, methyl triflate, andtriflic acid had a different color (red) than the polymers from thefirst four cationic initiators.

[0033] The polymers from the cationic polymerization of benzoxazinemonomer BA-a were analyzed by Fourier transform infrared spectroscopyalong with a thermally cured polybenzoxazine from the same monomer.These results indicated substantially similar infrared spectra whenbisphenol A-based benzoxazine was used. The polymers compared to themonomer showed the dramatic change in the peaks in the region of 1,000to 1,350 cm⁻¹, arising from the CH₂ wagging (1327 cm⁻¹ and 1305 cm⁻¹),C—O—C asymmetric stretching (1233 cm⁻¹), C—N—C asymmetric stretching(1159 cm⁻¹), and C—O—C symmetric stretching (1031 cm⁻¹) of the oxazinering, respectively, as well as the decrease of the peak resolution,indicate the opening of the oxazine ring and the polymerization of themonomer into oligomers and polymers.

[0034] There are significant spectral differences between the thermallypolymerized polybenzoxazine and the PCl₅ initiated polybenzoxazines. Inthe spectrum of BA-a monomer the peaks centered at 1500 cm⁻¹ and 949cm⁻¹ are major characteristics of the tri-substituted benzene ring inthe benzoxazine structure, corresponding to the in-plane C—C stretchingand the out-of-plane C—H deformation of tri-substituted benzene ring,respectively. These two peaks almost disappear in the thermallypolymerized polybenzoxazine spectrum, meanwhile a new peak centered at1489cm⁻¹, representing tetra-substituted benzene ring mode, appears. Theresults are in accordance with the well-accepted thermal polybenzoxazinestructure shown as below:

[0035] However, with some cationic initiators, here PCl₅ has been chosenas an example, these two peaks at 1500 cm⁻¹ and 949 cm⁻¹ remainunaffected except for the peak broadening, which indicates that in thepolybenzoxazine structure obtained, the majority of the benzene ringsshould still be trisubstituted instead of tetrasubstituted. Further NMRresults are in support of this hypothesis.

[0036] Polymers for size exclusion chromatographic (SEC) analysis werecationically polymerized from the BA-a monomer using chloroform as asolvent at about 23° C. using PCl₅ initiator and a mole ratio of 1:100initiator:monomer. Portions of the reactants were withdrawn after 1, 2,2.5, 3.5, 5, and 7 hours of reactions and poured into an excess ofmethylol to cause precipitation. The precipitated material was recoveredand analyzed by SEC for differences in molecular weight as a function ofreaction time. As is well-known in SEC analysis, short retention timesindicate higher molecular weight, while long retention times indicatelower molecular weight. A peak at 29.3 minutes was associated with thebenzoxazine monomer. A new peak occurred between 21 and 29 minutes asthe polymerization occurred. Simultaneously the peak at 29.3 minutesdecreased in size. This is consistent with theories on polymerization.

[0037] An NMR experiment was conducted comparing polymer from a cationicpolymerization, using PCl₅ as the initiator in deuterated chloroform asa solvent, to a thermally polymerized polymer. The reaction temperaturefor the cationic polymerization was 40° C., the reactor was a sealed NMRtube, and the material was evaluated after 10 minutes, 30 minutes, 1hour, and 2 hours to see which NMR peaks were being generated ordepleted as the monomer was cationically polymerized. The monomer peaksat 2.6, 3.9, and 4.75 ppm gradually disappeared, while new peaks at 1.95and 3.65 ppm appeared, and a broadening of the peak at 2.2 ppm occurredindicating less molecular rearrangement occurred ongoing from monomer topolymer under cationic conditions. Based on these results (proton NMR)along with carbon ¹³NMR and FTIR results, the repeat structure of thethermally polymerized polymer in these studies is theorized to bepredominantly tetrasubstituted phenolic structure, e.g.,

[0038] while the cationically polymerized samples is predominantly thephenoxy repeating units of the structure

[0039] In conclusion, the experimental data indicates the cationicallyinitiated polymerization benzoxazine monomers can occur at substantiallylower temperatures than thermal polymerization and produces polymers ofdifferent microstructure than thermal polymerizations. Further, thesepolymers can have substantially higher char yields 62\29 or an increaseof 200 percent over the char yield of the thermally polymerizedpolymers.

[0040] The polymers from benzoxazines are useful as precursors for charyielding material (e.g., precursors to aircraft brake pads). They arealso useful as temperature and flame resistant polymers for electricalcomponents, planes, cars, buses, etc.

[0041] A polybenzoxazine material has the following characteristics,making it desirable as a microelectronic or optoelectronic underfillmaterial:

[0042] (i) Low moisture uptake;

[0043] (ii) Low volume shrinkage during curing;

[0044] (iii) Good film-forming properties (i.e., thickness control andtackiness at room temperature);

[0045] (iv) A low coefficient of thermal expansion (CTE).

[0046] The following table compares the properties of a typicalpolybenzoxazine compound with a typical epoxy: TABLE 3 TypicalPolybenzoxazine- Property Based Film Typical Epoxy Curing shrinkage (%)0 or expansion 3-4 Moisture uptake (%) 1.5  3 Modulus (GPa) 4 2-3Tensile strength (MPa) 130 120 Tg (° C.) 170 150 Impact strength (J/m)30  30 CTE (ppm/° C.) 55  65

[0047] Different film structures can be designed to meet applicationrequirements. Film structures according to embodiments of the inventionare shown in FIGS. 1a-1 e. In each case, a film 2 a-e is providedbetween two removable sheets 1. FIG. 1a illustrates a film 2 a, which isa polybenzoxazine-based film as hereinbefore described. FIG. 2billustrates a polybenzoxazine-based film 2 b includinguniformly-dispersed filler particles 4. FIG. 2c illustrates apolybenzoxazine-based film 2 c including anisotropic electricallyconductive particles 3. FIG. 2d illustrates a polybenzoxazine-based film2 d including electrically conductive columnar components 5. FIG. 2eillustrates a polybenzoxazine-based film having multiple layers 2 e withcomponents 3 therein. The components 3 typically have a lower CTE thanthe polybenzoxazine material of the layers 2 e. More of the components 3are located in an upper one of the films 2 e than in lower ones of thefilms 2 e so that the upper film has a lower coefficient of thermalexpansion due to the higher number of components 3 than lower ones ofthe films 2 e.

[0048]FIG. 2a illustrates an optoelectronic package 8 a and anoptoelectronic die 9 which is attached to a package substrate 6 by meansof an adhesive film 7. The film 7 may, for example, be the film 2 aillustrated in FIG. 1a. Film 7 shrinks by a relatively small amountduring curing so that the position of the die 9 is substantiallymaintained relative to the substrate 6. Such a feature is desirable whencoupling light from another component, which is at a predetermineddistance from the package substrate 6, to a feature 9 a on the die 9. Amicroelectronic package can be assembled n a similar manner by replacingthe optoelectronic die in 9 with a microelectronic die.

[0049]FIG. 2b illustrates an electronic assembly 8 b having asemiconductor die 13 attached to a package substrate 10 with a film 11that may be the same as the film 2 d illustrated in FIG. 1d. The film 11thus includes a plurality of columnar members 14. Each member 14 has anupper end in contact with a respective contact 12 on the die 13, and alower end in contact with a respective pad 15 of the package substrate10. The die 13 is in electric communication with the package substrate10 through the members 14. A lower surface of the die 13 adheres to anupper surface of the material 11, and an upper surface of the packagesubstrate 10 adheres to a lower surface of the material 11. The material11 electrically insulates the members 14 from one another.

[0050]FIG. 2c illustrates an electronic assembly 8 c including asemiconductor die 20 which is attached to a package substrate 16 througha film 18. The die 20 has a plurality of contacts 21 on a lower surfacethereof, and the package substrate 16 has a plurality of pads 24 on anupper surface thereof. The film 18 includes a polybenzoxazine material19 having an upper surface attached to a lower surface of the die 20,and a lower surface attached to an upper surface of the packagesubstrate 16. The film also has columnar members 22 in the material 19.Each member 22 establishes an electrical path between a die and itsrespective substrate, and has a respective upper end contacting arespective one of the contacts 21 and a respective lower end contactinga respective one of the pads 24. The film 18 also has a plurality offiller particles 17 in the material 19. The particles 17 have a lowerCTE than the material 19, and thus decrease the overall CTE of the film18. More of the particles are included near the die 20 than near thepackage substrate 16, so that the CTE of the film 18 is lower near thedie 20 than near the package substrate 16. The die 20 has a lower CTEthan the package substrate 16. The CTE of the film 18 closely matchesthe CTE of the package substrate 16 near the package substrate 16, andclosely matches the CTE of the die 20 near the die 20. A similar packagecan be constructed without columns in the film. It may, for example, bepossible to locate the film of FIG. 1e on a substrate having bumps orother contacts thereon, or a substrate without bumps but with bumps onthe die. It may also be possible to use the film of FIG. 1e between adie and a substrate where both die and substrate have facing bumps, sothat the bumps on both the die and the substrate penetrate the film.

[0051] Most of polybenzoxazine-based film monomers are solids at roomtemperature. But they normally have low melt viscosity at slightlyelevated temperatures such as 70-80° C. Cured pure polybenzoxazine-basedfilm materials also have a lower CTE than a typical epoxy material,which allows lower filler loading to reach the same CTE value as atypical epoxy. The low melt viscosity and lower filler loadingfacilitate the material flow and wet on contact surfaces, and thereforeprovide a wider process window. Additionally, the lower moisture uptakeof the polybenzoxazine-based film material is expected to have betterperformance of a finished package during moisture testing. There is alsoa slight volume increase during the cationic ring opening polymerizationof the polybenzoxazine material. This volume expansion can effectivelyreduce residual stresses during the cooling of the entire package.

[0052] Considering that polybenzoxazine-based film monomers can beeasily made through Mannich reaction from a phenolic derivative,formaldehyde, and a primary amine, it can be seen that apolybenzoxazine-based film can be synthesized from a wide selection ofraw materials consisting of phenolic derivatives and primary amines. Themolecular structure of polybenzoxazine-based films offers superb designflexibility to allow for the properties of the cured materials to becontrolled for the specific requirements of a wide variety of individualapplications.

[0053] While certain exemplary embodiments have been described and shownin the accompanying drawings, it is to be understood that suchembodiments are merely illustrative and not restrictive of the currentinvention, and that this invention is not restricted to the specificconstructions and arrangements shown and described since modificationsmay occur to those ordinarily skilled in the art.

What is claimed:
 1. An electronic assembly, comprising: a substrate; afilm adhered to an upper surface of the substrate, the film including apolybenzoxazine polymer; and an electronic die adhered to an uppersurface of the film.
 2. The electronic assembly of claim 1, wherein theelectronic die is one of a semiconductor die and an optoelectronic die.3. The electronic assembly of claim 1, wherein the film includesparticles having a different CTE than the polymer.
 4. The electronicassembly of claim 3, wherein the particles are substantially uniformlydispersed.
 5. The electronic assembly of claim 3, wherein more particlesare included in one layer of the film than another layer of the film 6.The electronic assembly of claim 5, wherein the particles provide thefilm with a different CTE in one layer than in another layer.
 7. Theelectronic assembly of claim 6, wherein the CTE of the film is lowernear the die.
 8. The electronic assembly of claim 1, wherein the filmincludes a plurality of members, each member electrically connecting arespective contact on the die with the respective pad on the substrate.9. The electronic assembly of claim 1, wherein the polymer is derivedfrom 2H-1,3-dihydrobenzoxazine monomers, comprising; repeating unitshaving the structure

or combinations thereof, wherein if Structure II is present, the polymermust include at least 2, 5, 10, or 15 mole percent of repeating units ofStructures I, Ib, or IIb; wherein each R₁ is one or more groups selectedfrom hydrogen, an alkyl of 1 to 80 carbon atoms, aromatic, alkylsubstituted aromatic, or aromatic substituted alkyls having from 6 to 20carbon atoms, mono or poly fluorine substituted alkyls of 1 to 20 carbonatoms, mono or poly substituted compounds having at least one aromaticring and 6 to 20 carbon atoms, or phenolic compounds having from 6 to 20carbon atoms and/or a connecting group forming a branch in the polymer;R₂ is an alkyl of 1 to 40 carbon atoms, an aromatic substituted alkyl oralkyl substituted aromatic of 6 to 40 carbon atoms, mono or polyfluorine substituted alkyl of 1 to 20 carbon atoms, a mono or polyfluorine substituted compound with at least one aromatic ring and 6 to20 carbon atoms, an amine of 1 to 40 carbon atoms including optionallypolyamines and aromatic, alkyl substituted aromatic or aromaticsubstituted alkyl amines of 6 to 40 carbon atoms or a connecting groupselected from the R₂ groups connected to another polymer frombenzoxazine monomers; and wherein R₃ is H or R₂; and X is a counterionsuch as a halogen and p is an integer from 0 to
 3. 10. The electronicassembly of claim 9, wherein the polymers are derived from the ringopening polymerization of one or more 2H-1,3,-dihydrobenzoxazinemonomers.
 11. The electronic assembly of claim 10, wherein the one ormore 2H-1,3,-dihydrobenzoxazine monomers include at least 25 mole % ofone or more monomers having the formula

wherein each R₁ individually is one or more groups selected fromhydrogen, alkyls of 1 to 10 carbon atoms, aromatic, alkyl substitutedaromatic, or aromatic substituted alkyl of 6 to 20 carbon atoms, mono orpoly fluorine substituted alkyls of 1 to 20 carbon atoms, mono or polyfluorine substituted compounds having at least one aromatic ring and 6to 20 carbon atoms or a benzoxazine from a phenolic compound of 6 to 20,n is from 1 to 4; wherein each R₂ is an alkyl of 1 to 10 carbon atoms,an aromatic, alkyl substituted aromatic or aromatic substituted alkyl of6 to 20 carbon atoms, or an amine of 1 to 10 carbon atoms, or abenzoxazine of 9 to 20 carbon atoms; and wherein R₃ and p are as definedabove.
 12. The electronic assembly of claim 11, wherein the R₁ group ofthe formula A is ortho to the oxygen of the benzoxazine.
 13. Theelectronic assembly of claim 11, wherein the R₁ group of the Formula Ais meta to the oxygen and para to the CH₂—N—(R₂).
 14. The electronicassembly of claim 11, wherein the R₁ group of formula A is para to theoxygen.
 15. The electronic assembly of claim 9 having a number averagemolecular weight of at least 5,000.
 16. The electronic assembly of claim9, wherein the polymer is the reaction product of reacting at least one2H-1,3,-dihydrobenzoxazine monomer with a cationic polymerizationinitiator.
 17. The electronic assembly of claim 16, wherein the cationicpolymerization initiator comprises PCl₅, PCl₃, POCl₃, TiCl₅,(C₆H₅)₃C⁺(SbCl₆)—, SbCl₅, methyl triflate, methyl tosylate, triflicacid, or aluminum phthalocyanine chloride or combinations thereof. 18.The electronic assembly of claim 16, wherein the2H-1,3-dihydrobenzoxazine monomer comprises at least 25 mole percent ofone or more monomers having the formula

wherein R₁ individually is hydrogen, one or more groups selected fromalkyls of 1 to 10 carbon atoms, an aromatic, alkyl substitutedaromatics, or aromatic substituted alkyls of 6 to 20 carbon atoms, monoor poly fluorine substituted alkyls of 1 to 20 carbon atoms, mono orpoly fluorine substituted compounds having at least one aromatic ringand 6 to 20 carbon atoms, or a benzoxazine from a phenolic compound of 6to 20, n is from 1 to 4; wherein each R₂ is an alkyl of 1 to 10 carbonatoms, an aromatic, alkyl substituted aromatic or aromatic substitutedalkyl of 6 to 20 carbon atoms or an amine of 1 to 10 carbon atoms or abenzoxazine of 9 to 20 carbon atoms; and wherein R₃ is H or R₂ and p isan integer between 0 and
 3. 19. A method of constructing an electronicassembly, comprising: adhering an electronic die to a substrate with afilm including polybenzoxazine.
 20. The method of claim 19, wherein thefilm includes a plurality of electrically conductive members, eachmember electrically connecting a respective contact on the die with arespective pad on the substrate.
 21. The method of claim 19, wherein thefilm is formed by reacting a cationic polymerization initiator with2H-1,3,-dihydrobenzoxazine monomers at a temperature from about −100° C.to about 250 C.
 22. The method of claim 21, wherein the cationicpolymerization initiator comprises PCl₅, PCl₃, POCl₃, TiCl₅,(C₆H₅)₃C⁺(SbCl₆)—, SbCl₅, methyl triflate, methyl tosylate, triflicacid, or aluminum phthalocyanine chloride or combinations thereof. 23.The method of claim 21, wherein the 2H-1,3,-dihydrobenzoxazine monomersforming the polymer comprise at least 25 mole percent of one or moremonomers of the formula

wherein R₁ individually is hydrogen, one or more groups selected fromalkyls of 1 to 10 carbon atoms, an aromatic, alkyl substitutedaromatics, or aromatic substituted alkyls of 6 to 20 carbon atoms,aromatic ring and 6 to 20 carbon atoms mono or poly fluorine substitutedalkyls of 1 to 20 carbon atoms, mono or poly fluorine substitutedcompound having at least one or a benzoxazine from a phenolic compoundof 6 to 20, n is from 1 to 4; wherein each R₂ is an alkyl of 1 to 10carbon atoms, an aromatic, alkyl substituted aromatic, or aromaticsubstituted alkyl of 6 to 20 carbon atoms, or an amine of 1 to 10 carbonatoms, or a benzoxazine of 9 to 20 carbon atoms; and wherein R₃ is H orR₂ and p is an integer from 0 to
 3. 24. The method of claim 21, whereinat least 20 mole % of the repeat units of the polymer arc of structureI.
 25. A method of constructing an electronic assembly, comprising:locating a film between a substrate and an electronic die; and curingthe film by first heating the film and then allowing the film to cool,the film having less than 0.2% shrinkage during curing.
 26. The methodof claim 25, wherein the film expands by at least 0% during curing. 27.The method of claim 25, wherein the film has a modulus of at least 1GPa, a strength of at least 80 MPa, and an impact strength of at least15 J/m.
 28. The method of claim 27, wherein the film has a modulus of atleast 4 GPa, a strength of at least 130 MPa, and an impact strength ofat least 30 J/m.
 29. The method of claim 25, wherein the film, withoutany filler material, has a CTE of less than 70 ppm/° C.
 30. The methodof claim 25, wherein the film includes a polybenzoxazine polymer.